1. Field of the Invention
The invention relates to a process for preparing polydiorganosiloxanes, each of which has an Si-bonded hydroxyl group at one terminus and an Si-bonded, optionally substituted, alkoxy group at the other terminus.
2. Background Art
DE-C 42 34 959 (Th. Goldschmidt AG) describes the ring-opening polymerization of hexamethylcyclotrisiloxane with lithium alkoxide and termination of the reaction using a chloro-, bromo-, or acyloxysilane. From a safety point of view, the preparation of lithium alkoxide from the appropriate alcohol with metallic lithium is problematic, as is the use of ethers as solvents for the alkoxide: such ethers are prone to generation of involatile peroxides. Furthermore, the handling of lithium alkoxide is costly and inconvenient owing to its sensitivity to hydrolysis.
U.S. Pat. No. 3,445,426 (Dow Corning Corporation) discloses the polymerization of hexaorganocyclotrisiloxane in the presence of catalytic amounts of a penta-coordinated silicon catalyst or alkali metal silicate, ammonium silicate or phosphonium silicate. However, the catalysts have the disadvantage that they are costly and inconvenient to prepare and very moisture-sensitive. The initiator specified for the ring-opening polymerization is alcohol in combination with water, which always leads to polymer mixtures, i.e. OH-terminated polymers and monoalkoxy-terminated polymers.